The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust treatment technology for reducing CO and HC emissions is an oxidization catalyst (“OC”) device. The OC includes a flow-through substrate and a catalyst compound applied to the substrate. The catalyst compound of the OC induces an oxidization reaction of the exhaust gases once the OC has attained a threshold or light-off temperature. One type of exhaust treatment technology for reducing NOx emissions is a selective catalyst reduction (“SCR”) device that may be positioned downstream of the OC device. The SCR device includes a substrate, where a SCR catalyst compound is applied to the substrate. A reductant is typically sprayed into hot exhaust gases upstream of the SCR device. The reductant may be a urea solution that decomposes to ammonia (NH3) in the hot exhaust gases and is absorbed by the SCR device. The ammonia then reduces the NOx to nitrogen in the presence of the SCR catalyst. However, the SCR device also needs to reach a threshold or light-off temperature to effectively reduce NOx. During a cold start of the engine, the OC and the SCR have not attained the respective light-off temperatures, and therefore generally may not effectively remove CO, HC, and NOx from the exhaust gases.
One approach for increasing the effectiveness of the OC and the SCR devices involves having the engine operate at a higher temperature, which in turn also raises the temperature of the exhaust gases. However, this approach involves the engine operating at a lower level of efficiency to create the hotter exhaust gas, which results in greater fuel consumption. Accordingly, it is desirable to provide an efficient approach to increasing the temperature of the exhaust gases upstream of the OC and the SCR devices.